Physiologic fluid pressure sensor head

ABSTRACT

A pressure sensing head for measuring physiologic fluid pressure through an elastic body membrane, which is composed of: a membrane depressor-element with a membrane-depressing face adapted to engage the surface of the membrane; a supporting structure containing a pressure-development cavity, a port for introducing gas under pressure into the pressure-development cavity and a port for measuring the gas pressure developed within the pressure-development cavity; and a feedback chamber formed in the supporting structure and adapted in a manner such that the depressor-element is free to move relative to the supporting structure to vary the free volume of the feedback chamber. The feedback chamber is either an integral part of or connected to the pressure-development cavity and is connected through a passage in the depressor element to the membrane-depressing face of the depressor element. The membrane-depressing face is adapted so that the gas introduced into the pressure-development cavity will be discharged into the atmosphere in a manner such that the discharging gas is throttled by the disposition of the membrane relative to the membrane-depressing face.

United States Patent [54] PHYSIOLOGIC FLUID PRESSURE SENSOR HEAD 12Claims, 10 Drawing Figs.

[52] U.S. Cl 128/105 E, 73/80, 128/2 R, 128/205 N [51] lnt.Cl A6lb 5/02[50] Field of Search 128/2, 2.05

E, 205 N, 205 R; 73/80 [56] References Cited UNITED STATES PATENTS3,099,262 7/1963 Bigliano 128/205 E 3,150,521 9/1964 Mackey et al. 73/80OTHER REFERENCES Block Engineering Inc, Model 10 Durham Tonometer 2pp.,Feb 10,1966.

Primary Examiner-William E. Kamrni A!t0meyHerbert M. Wolfson ABSTRACT: Apressure sensing head for measuring physiologic fluid pressure throughan elastic body membrane, which is composed of: a membranedepressor-element with a membrane-depressing face adapted to engage thesurface of the membrane; a supporting structure containing apressuredevelopment cavity, a port for introducing gas under pressureinto the pressure-development cavity and a port for measuring the gaspressure developed within the pressure-development cavity; and afeedback chamber formed in the supporting structure and adapted in amanner such that the depressor-ele' ment is free to move relative to thesupporting structure to vary the free volume of the feedback chamber.The feedback chamber is either an integral part of or connected to thepressure-development cavity and is connected through a passage in thedepressor element to the membrane-depressing face of the depressorelement. The membrane-depressing face is adapted so that the gasintroduced into the pressure-development cavity will be discharged intothe atmosphere in a manner such that the discharging gas is throttled bythe disposition of the membrane relative to the membranedepressing face.

mum mm 3.628, 526

SHEET 2 BF 2 T fbi INVENTOR ROBERT P. IBIGLIANO ATTORNEY PHYSIOLOGICFLUID PRESSURE SENSOR HEAD BACKGROUND OF THE INVENTION This inventionrelates to an improved device for rapidly and accurately measuringphysiologic fluid pressure. More particularly it relates to an improvedhead for a pressure sensing device adapted to measure the pressure ofbody fluids, such as blood pressure, interocular pressure and the like,through the walls of an elastic body membrane. Still more particularlyit relates to an improved head for a pneumatic aplanation-type pressuresensing device, such as that described in U.S. Pat. No. 3,099,262. Thepresent invention is particularly suited for use as a tonometer head inthe measurement of interocular pressure but its use is not limited tothat use.

Aplanation-type pressure sensing devices operate by first flattening agiven area a on the surface of the membrane encasing the fluid, and thenexerting a gentle but steadily increasing force, over that area, untilthe external applied pressure overcomes the internal supporting pressureof the fluid, at which point a pressure discontinuity occurs. By firstflattening a given area of the membrane, supporting effects due to thestiffness of neighboring body tissue is minimized so that the measuredexternal pressure is related to only the internal fluid pressure and tothe stiffness of the body tissue directly within the area of contact.Various methods have been devised to compensate for this remainingmembrane stiffness. Most are based on the fact that the area of contactand, therefore, the amount of membrane involved is a constant. Oneparticularly useful device for achieving this compensation is thepneumatic pressure sensing head described in U.S. Pat. No. 3,099,262,where the measured pressure is related to the actual pressure by aconstant of proportionality which is a function only of the dimensionsof the pressure sensing head. This instrument functions quite well, butthere are some inherent problems in its use which have heretofor limitedits ac curacy.

It is characteristic of this type of pressure sensing device that it ismanually held. Since the forces involved are of such small magnitude,and since hand movements are of such a gross nature, reproducibleflattening and hence the pressure discontinuity end point have beendifficult to achieve. Another problem is found in the fact thatdifferent membranes in different people have different surfacecharacteristics, so that a different pressure must be exerted to flattena given area of the membrane.

These difficulties have been generally recognized and several methodshave been used to correct them. The simplest and most obvious is torepeat the measurement several times to average out the effect of anyunsteadiness in the hand of the user as well as variations in thecharacter of the surface of the different membranes. A moresophisticated method is discussed in US. Pat. No. 3,299,882 where agimbled selfconforming mount is used to accommodate the pressure sensingdevice to the body site. Both of these solutions have obvious defects interms of the amount of time required to make the measurement, the amountof operator discretion involved and the sheer clumsiness of theinstrument.

Accordingly, it is an object of this invention to provide an improvedpressure sensing device wherein the pressure sensing element thereof,once set upon the membrane, automatically seeks a predetermined area ofcontact independent of the operator and maintains this stable positionwhile the measurement is being made.

Another object of the present invention is the provision of a novelmount for the pressure sensing element such that once the element is instable contact with the membrane the motion of the pressure sensingelement is independent of gross movements ofthe mount.

A still further object of this invention is the provision of a novelaplanation pressure sensing device wherein the area of flatteningachieved thereby remains constant and independent of structuraldifferences in different parts of the membrane or in the membranes ofdifferent subjects.

SUMMARY OF THE INVENTION Accordingly these objects are achieved byproviding, in a pneumatic aplanation pressure sensing device, apressure-sensor mounting means capable of providing a feedback force forurging the pressure sensing face into ifirm contact with the surface ofthe membrane, with continually increasing force applied independent ofthe operator, until the force, due to the internal pressure of the fluidacting over the contact area, is balanced by the feedback force, and anequilibrium condition is established such that the pressure acting onthe sensor corresponds to the internal fluid pressure.

Generally, the pressure sensing device comprising: a membrane-depressorelement with a membrane-depressing face adapted to engage the surface ofthe membrane; a supporting structure containing a pressure-developmentcavity, a port for introducing gas under pressure into thepressure-development cavity and a port for measuring the gas pressuredeveloped within the pressure-development cavity; and a feedback chamberformed in the supporting structure in a manner such that thedepressor-element is free to move relative to the supporting structureto vary the free volume of the feedback chamber. The feedback chamber iseither an integral part of or connected to the pressure-developmentcavity and is connected through a passage in the depressor-element tothe membrane-depressing face of the depressor-element. Themembrane-depressing face is adapted so that the gas introduced into thepressure-development cavity will be discharge into the atmosphere in amanner such that the discharging gas is throttled by the disposition ofthe membrane relative to the membranedepressing face. In one embodimentthe feedback chamber is in the form of a cylinder and thedepressor-element is in the form of a piston, free to move within thecylinder without becoming disengaged from it. In another embodiment thedepressor element is attached to a flexible diaphragm which is itselfattached to an annular flange extending above the supporting structureso that the feedback chamber is formed between the diaphragm and uppersurface of the supporting structure. In one embodiment the depressorelement is restrained from moving in. a direction other than in adirection perpendicular to the membrane-depressing face. In anotherembodiment the depressor element is made from a flexible material sothat the membrane-depressing face can move in any direction. In apreferred embodiment the membrane-depressing face has an annulardepression in its surface concentric with the opening connected to thefeedback chamber and separated from the opening by a ridge with asurface recessed from the membrane-depressing face. The annulardepression is connected to the rear of the depressor element byair-escape ports so that gas entering the pressuredevelopment cavitywill pass through the opening in the center of the membrane-depressingface and exhaust into the atmosphere through the annular depression andthe air-escape ports without completely disrupting contact between themembrane and the membrane-depressing face.

The advantages and operation of the present invention can best beunderstood by reference to the following figures:

FIG. 1a is a side elevation cross-sectional view of a preferredembodiment of the pressure sensing head of the present invention;

FIG. 1b is a side elevation cross-sectional view of a portion of thepressure sensing head illustrated in FIG. 1a, showing a modification ofthe support structure for the membranedepressing face.

FIG. 2 is a side elevation cross-sectional view of another preferredembodiment of the pressure sensing head of the present invention;

FIG. 3 is a side elevation cross-sectional view of a third preferredembodiment of the pressure sensing head of the present invention;

FIG. 4 is a side elevation cross-sectional view of a fourth preferredembodiment of the pressure sensing head of the present invention;

FIG. 5a, 5b and 5c illustrate how the pressure sensing head of thepresent invention engages the membrane surface and is brought insuccessive stages into stable and fixed area contact with the membrane;

FIGS. 6a and 6b provide a comparison between measurements made with aninstrument incorporating the present invention to measurements made witha conventional instrument.

DETAILED DESCRIPTION OF THE DIAGRAMS In FIG. 1a the supporting structureis divided into two parts, a first body part 11 and a second body part12. The first body part 1 1 is provided with a gas pressure-developmentcavity 13, a first port 14 for introducing gas under pressure into thepressure-development cavity at a substantially constant rate of flow,and a second port 15 for measuring the pressure developed within thepressure-development cavity. As discussed in the prior art, specificallyU.S. Pat. No. 3,099,262, the gas supply port 14 is provided with arestriction 14a for preserving the fixed flow rate of gas delivered tothe cavity. Port 14 is externally connected to some source of fluidsupply, typically a gas at 30 p.s.i.g. As illustrated in FIG. 1, the gaspressure-development cavity 13 is in the form of a chamber formed withinthe first body part and connected to the upper surface of the first bodypart by a passage 16. As can be seen from FIG. 2, however, thepressure-development cavity 13 can be in the form of a cavity, recessedin the upper surface of the first body part rather than a completelyenclosed chamber connected by a passage to the upper surface. The shapeor location of the cavity are not critical and will be determined by theneeds of the instrument and by manufacturing considerations. Anyconvenient pressure measuring device can be connected to pressuremeasurement port 15. The only criterion for the measuring device is thatit must be sensitive enough to detect the small pressure variations thatoccur in the cavity.

The first and second body parts of the supporting structure can be madefrom any rigid, easily formed, material, like a metal or a plastic.Typically stainless steel or Lucite acrylic resin can be used. The firstbody part 11 is adapted to be attached to the second body part 12 insome substantially leaktight arrangement. One simple way to achieve thisis to threadedly join the two parts. In the configuration illustrated inFIGS. 1 through 4 this would be accomplished by threading the first bodypart 11 and internally tapping the second body part 12 so as to bethreadedly joined to the first body part.

The second body part 12 is in the form of the frustum of a hollow cone.The narrowest portion of this frustum is adapted to engage themembrane-depressor element 17 in a substantially friction freearrangement. As illustrated in FIG. 1 the membrane-depressor elementconsists of a disc 18, with a membrane-depressing face 19, and acylindrical support tube 20 to which the disc is attached. The supporttube, in this particular instance, consists of two cylindrical parts, atop part 20a attached to the disc, and a bottom part 20b attached to thetop part. The second body part 12 has a channel 21 formed in it so thatthe support tube of the membrane-depressor element fits into the secondbody part like a piston in a cylinder. The second body part is hollowedout into a chamber, called the feedback chamber 22, which is connectedto the pressure-development cavity 13 by passage 16. As illustrated, thebottom part 20b of the support tube has a larger diameter than the upperpart 20a, and also a larger diameter than channel 21 and passage 16, sothat while the membranedepressor element is free to move relative to thesupporting structure it is restricted in its movement by the fact thatthe bottom part of the support tube 20b is constrained within thefeedback chamber. The supporting structure and the membrane-depressorelement are disposed so that the former engages the latter in asubstantially leaktight and substantially friction free arrangement.This can be accomplished by providing a narrow clearance between the twoparts, as illus trated, by using a Teflon fluorocarbon gasket, or byusing any conventional substantially friction-free sealing means knownto those skilled in the art. The only criterion involved in the designis that the membrane-depressor element is constrained to move relativeto the supporting structure in a direction perpendicular to the plane ofthe membrane-depressing face 19, without becoming disengaged from thesupporting structure and in a manner such that motion of themembrane-depressor element will vary the free volume in the feedbackchamber 22.

As a modification to a structure shown in FIG. Ia, FIG. 1b shows therigid support tube 20a connected to disc 18 by a flexible sleeve 20c.The flexible sleeve allows motion of the membrane-depressing face indirections other than perpendicular to the plane of the face while therigid support tube 20a still substantially restricts motion of themembranedepressor element to motion in a direction perpendicular to theplane of the membrane-depressing face.

The structure is designed so that gas introduced into thepressure-development cavity 13 will pass through the feedback chamber 22into the internal passage 24 in the membrane-depressor element 17, fromwhich it will be discharged into the atmosphere.

The design of the membrane-depressing face is an important aspect ofthis instrument. The design is according to the teaching of U.S. Pat.No. 3,099,262 and as such is not part of the present invention, butrather an essential feature in the design of any pneumatic aplanaticpressure sensing device. The membrane depressing face is adapted toengage and flatten the surface of a membrane 25 into conformity with thesurface of the membrane-depressing face. The membranedepressing face isfurther adapted so that gas discharging form the pressure-developmentcavity passes through an opening 26 in its surface and is exhausted intothe atmosphere without completely disrupting contact between themembrane and the surface of the membrane-depressing face. Themembranedepressing face is still further adapted so that the discharginggas is throttled by the disposition of membrane 25 relative to theopening 26 in the membrane-depressing face. As illustrated in FIGS. 1through 4, this can be accomplished by providing the membrane-depressingface with an annular depression 27 circumscribing opening 26 andseparated from it by a ridge 28. The annular depression is thenconnected to the rear surface of the disc 18, by a series of air-escapeports 29. The surface of ridge 28 separating the annular depression 27from opening 26 is recessed from the surface of the membrane-depressingface by a predetermined distance. In accordance with the teaching ofU.S. Pat. No. 3,099,262 the ridge is recessed by about 0.0010 inch. Gasdischarging from the passage 24 in the membrane-depressor element must,therefore, pass through the space betweenthe membrane 25 and the surfaceof the ridge 28, before being exhausted to the atmosphere, and isthrottled by the separation between them. Also following the teaching ofU.S. Pat. No. 3,099,262, the membrane-depressing face can be covered bya thin membrane (not shown) which for sanitary purposes separates thebody membrane from the surface of the membrane-depressing face withoutaffecting the measurement. The membranedepressor element need not bemade in one piece or from one material. In fact, as shown, to positionthe parts the membrane-depressor element must be made in several partswhich are attached after the support tube has been inserted through theopening 21. The parts can be made from any relatively rigid materialsuch as Delrin acetal resin.

Before discussing the operation of this instrument, the otherembodiments depicted in FIGS. 2 through 4 will be discussed, since theoperation of all four embodiments is similar. In FIG. 2 themembrane-depressor element 17 no longer acts as a piston in the feedbackchamber 22. Instead a flexible diaphragm 30, supported on an annularflange 31 extending above the first body part 11, is provided. Theregion between the diaphragm 30 and the upper surface of the first bodypart 1 l is the feedback chambenThe diaphragm 30 has an opening 32 inits center and the membrane-depressor element is attached to thediaphragm so that the internal passage 24 in the support tube isconnected to the opening 32 in the diaphragm. The gas introduced intothe pressure-development cavity still passes through the feedbackchamber and is discharged into the atmosphere through themembrane-depressor element. Since the diaphragm is flexible and sincethe second body part 12 of the supporting structure is not adapted toengage the support tube 17 in a manner such as to restrain its movement,the membrane-depressor element is not constrained to move in a directionperpendicular to the plane of the membranedepressing face. Themembrane-depressor element can move in a direction perpendicular to theplane of this face, however, and if it does, then its movement will varythe free volume in the feedback channel. It must be noted, however, thatthe membrane-depressor element can also be moved by deforming thediaphragm without changing the free volume in the feedback chamber. Thenonvolume changing motion does not affect the feedback action discussedbelow but will aid in aligning the membrane-depressing face with thesurface of the membrane. In the normal situation, both volume changingand nonvolume changing motion of the membrane-depressor element willoccur and both feedback and alignment action will be present.

The diaphragm can be made from any flexible material such as a thinmetal sheet or an elastomeric material. One such material is Silasticsilicon rubber. If the support tube is also made from a flexiblematerial such as Silastic silicon rubber it will have a degree offlexibility which adds to the alignment action discussed above. The disc18, however, should still be made from a rigid material.

In FIG. 3 the diaphragm 30 has an annular deformation 33 circumscribingthe opening 32 in its center. The deformation has an outer diameter lessthan the diameter of the feedback chamber 22 and an inner diametergreater than the external diameter of the support tube 20 at the pointwhere it joins to the depression. The deformation is positioned so as toincrease the area of the feedback chamber; which is not absolutelynecessary. This bellowed diaphragm can be made from any flexiblematerial such as a metal or an elastomer.

As illustrated the feedback chamber 22 and the pressuredevelopmentcavity 13 are one and the same. The second body part 12 of thesupporting structure is once again adapted to engage themembrane-depressor element in a substantially friction free arrangementso that it is free to move only in a direction perpendicular to theplane of the membranedepressing face. This is accomplished, asillustrated, by using a Teflon fluorocarbon gasket 34. In this case thesupport tube 20 would be made from a rigid material instead of aflexible material as in the case of the support tube in FIG. 2, althoughthere is no reason why the portion of the support tube 20a nearest tothe disc 18 could not be made from a flexible material to preserve thealignment action.

FIG. 4 illustrates an embodiment of the invention which is similar tothat shown in FIG. 3 except that the feedback chamber 22 and thepressure development cavity 13 are no longer integral. Themembrane-depressor element is once again constrained in its movement,this time by providing the support tube 20 with a sleeve which, asillustrated, allows only a slight clearance between themembrane-depressor element and the supporting structure. Alternatively,the sleeve can be made from a material such as Teflon fluorocarbonadapted to contact the supporting structure in a substantially frictionfree arrangement.

As discussed above the novel feature of the present invention is to befound in the fact that once the membranedepressing face has been appliedto the membrane it automatically seeks a precise, reproducible positionrelative to the membrane, eliminating the need for successiveapplications of the instrument to the membrane to average out placementerrors. This self-correcting action is manifest in a tendency of themembrane-depressing face to move towards the surface of the membraneonce initial contact is made, and to resist any attempt to be withdrawn.Pneumatically, there are several possible explanations for thisattractive force. At the present time, however, it is believed that theproper explanation is that the feedback chamber provides a feedbackaction not present in prior art devices. And that this feedback actionaction supplies the force for the self-correcting action. In whatfollows the discussion will be limited to a discussion of a tonometerwhich is an instrument designed to measure interocular pressure throughthe cornea, since such instruments are particularly prone to theproblems discussed above. It is to be understood, however, that this isfor convenience and is not meant to limit the use of this invention.

Tonometers are usually manually applied to the eye, and, therefore, somecompensation for gross movement of the operators hand is necessary. Inthe present invention, for any gross motion of the operator's hand thereis a small compensating motion of the membrane-depressor element whichcauses the membrane-depressing face to sense a constant ocular area atall times after the equilibrium position has been established. Theoperation of this feedback action can be seen by reference to FIG. 5.When the membrane-depressing face 19 just touches the surface of the eye25, as shown in FIG. 5a, the area of contact with the eye A is verysmall. The measured pressure P, is also small, but this pressure isexerted on the area of the movable portion of the feedback chamber A andgenerates a force P,A,,, in the direction of the eye. In the case ofFIGS. 2 through 4, this area is the area of the diaphragm 30. In thecase of FIG. I it is the area of the lower surface of the support tube37. The only restraining force (neglecting scleral stiffness) is theinterocular pressure P, exerted on the small area of contact A,. Thus,there is a net force causing the membrane-depressor element to be movedtowards the eye, increasing the area of contact A with the eye as shownin FIG. 5b. The measured pressure P, increases and the force produced bythe feedback action also increases. At the same time the restrainingforce exerted by the eye increases, since the area of contact has alsoincreased. The net force continues to move the membrane-depressing facetowards the eye until an equilibrium position is reached. If there wereno stiffness effects (due to the sclera) this equilibrium would occur atthe point where P..A,.=P,A,,, As discussed in US. Pat. No. 3,099,262 themeasured pres sure is related to the interocular pressure by therelationship F e, where K is a constant related to the distance that thesurface of the ridge 28 is recessed from the surface of themembranedepressing face. Therefore P A =KP A and A =KA For a given headdesign, therefore, A will be a fixed area of contact, attainable withoutregard to any unsteadiness in the hand of the user, or geometricdifferences in the eye of the subject. If an attempt is made to removethe instrument from the eye, the area of contact A, decreases, whichcauses an increase in the net force towards the eye, and themembranedepressor element moves towards the eye. This is manifest in theobserved tendency of the instrument to stick on the eye.

Preferably the area'of the membrane-depressing face should be equal toor larger than the value of KA otherwise the depressor-element willcontinue to move towards the eye even after the membrane-depressing faceis fully seated on the eye. The invention is operable because anequilibrium position will occur due to the force exerted by the overdepressed sclera, but it is preferable and less painful to the patientif no over depression occurs. The value of K depends on the nozzleparameters and can be less than, equal to or greater than one. Again itis preferable if K is equal to or greater than one since therelationship between P, and P, will then be more nearly linear, but Kcan be less than one without affecting any aspect of this invention. IfK is greater than one, then the area of the membrane-depressing face incontact with the surface of the eye must be greater than the area of thediaphragm.

The implications of this feedback action can be seen with reference toFIG. 6, which compares the response of two tonometers. FIG. 6a shows theresponse of a tonometer employing the present pressure sensing head.FIG. 6b shows the response of a standard Durham tonometer. Bothtonometers are designed according to the teaching of U.S. Pat. No.3,099,262 and as such their sensitivity will depend upon the dimensionsof the membrane-depressing faces, but it is clear from a comparison ofthe two figures that there is a difference in the accuracy of the twoinstruments. In FlG. 6a four applications of the pressure head of thepresent invention were made to the eye and it can be seen that theinitial inflection points for each reading are all within 2 mm. Hg. ofone another. There is then efiectively only one inflection point in eachmeasurement. in FIG. 6b three applications of the Durham tonometer weremade to the eye. The two inflections at the right are within 2 mm. Hg ofoneanother, but the one at the left is falsely high. Since the two atthe right agree they are taken to be the valid reading, but as can beseen some discretion is needed in determining which readings are thecorrect readings, and this leads to an error not present when thepresent invention is used.

From the foregoing discussion, it will be apparent that the apparatus ofthis invention can be modified in numerous respects and that theforegoing discussion is not meant to limit the scope and/or use of thepresent invention.

What is claimed is:

1. In a pressure sensing head for measuring fluid pressure through amembrane including: a supporting structure containing apressure-development cavity; means for introducing gas at asubstantially fixed rate of flow into said cavity; means for measuringthe gas pressure in said cavity; and a membranedepressor elementsupported by said supporting structure, said membrane-depressor elementhaving a membranedepressing face with a flow throttling opening in itssurface and a passage connecting said flow throttling opening with saidcavity; the improvement comprising a variable volume feedback chamberwithin said supporting structure formed by saidmembrane-depressorelement and said pressure-development cavity, saidmembrane-depressor element being movably supported by said supportingstructure so that the volume of said feedback cavity is varied by themotion of said membrane-depressor element substantially perpendicular tothe surface of said membrane.

2. The pressure sensing head of claim 1 wherein said supportingstructure comprises a projection coacting with the sides of saidmembrane-depressor element to restrict its motion to motion in adirection perpendicular to the plane of said membrane-depressing face.

3. The pressure sensing head of claim 1 wherein said membrane-depressingface is provided with an annular depression concentric with said flowthrottling opening and separated therefrom by an annular ridge, thesurface of said ridge being recessed from the surface of saidmembrane-depressing face, and said membrane-depressor element isprovided with at least one gas escape port connecting said annulardepression to the atmosphere at a point removed from saidmembranedepressing face.

4. The pressure sensing head of claim 1 wherein:

a. said membranedepressor element further comprises a substantiallycylindrical support tube supporting said membrane-depressing face and b.said supporting structure comprises a projection coacting with the sidesof said cylindrical support tube in a substantially friction freearrangement to restrict its motion to motion in a directionperpendicular to the plane of said membrane-depressing face.

5. The pressure sensing head of claim 4 wherein:

a. said membrane-depressor element further comprises a disc supported bysaid support tube and containing said membrane-depressing face, saidmembrane-depressing face being provided with an annular depressionconcentric with said flow throttling opening and separated therefrom byan annular ridge, the surface of said annular ridge being recessed fromthe surface of said membranedepressing face, said disc being furtherprovided with at least one gas-escape port connecting said annulardepression to the atmosphere at the rear of said disc; and

b. said supporting structure comprises a first body part con tainingsaid cavity and a second body part, in the fonn of the frustum of ahollow cone, said second body part being supported by said first bodypart and the narrowest portion of said second body part coacting withsaid support tube in a substantially friction free arrangement torestrict its motion to motion in a direction perpendicular to the planeof said membrane-depressing face.

6. The pressure sensing head of claim 5 wherein said support tubecomprises a rigid part and a flexible part connecting said disc to saidrigid part, and wherein the second body part of said supportingstructure comprises a projection coacting with the rigid part of saidsupport tube to substantially restrict motion of said membrane-depressorelement to motion in a direction perpendicular to the plane of saidmembranedepressing face.

7. The pressure sensing head of claim 1 wherein said supportingstructure comprises an annular flange extending from said supportingstructure, and a flexible diaphragm supported in a substantiallyleaktight arrangement at its periphery by said annular flange to formsaid feedback chamber between said diaphragm and said supportingstructure, said membranedepressor element being supported by saiddiaphragm in a substantially leaktight arrangement, and said diaphragmbeing provided with an opening connecting said cavity to the passage insaid membrane-depressor element, whereby movement of saidmembrane-depressor element varies the volume of said feedback chamber bydeformation of said diaphragm.

8. The pressure sensing head of claim 7 wherein said membrane-depressorelement comprises a flexible, substantially cylindrical support tubesupporting said membrane-depressing face.

9. The pressure sensing head of claim 8 wherein said membrane-depressorelement further comprises a rigid sleeve surrounding said flexiblesupport tube over a portion of its length and wherein said supportingstructure comprises a projection coacting with said rigid sleeve tosubstantially restrict the motion of said membrane-depressor element tomotion in a direction perpendicular to the plane of saidmembranedepressing face.

10. The pressure sensing head of claim 7 wherein said supportingstructure comprises a projection coacting with the sides of saidmembrane-depressor element to restrict its motion to motion in adirection substantially perpendicular to the plane of saidmembrane-depressing face.

11. The pressure sensing head of claim 7 wherein said diaphragm has anannular deformation circumscribing the opening in its center, extendingaway from said feedback chamber, said deformation having an externaldiameter less than the diameter of said feedback chamber and an internaldiameter greater than the external diameter of said membrane-depressorelement.

12. The pressure sensing head of claim 7 wherein:

a. said membrane-depressor element comprises a disc containing saidmembrane-depressing face and a substantially cylindrical support tubesupporting said disc, said membrane-depressing face being provided withan annular depression concentric with said flow throttling opening andseparated therefrom by an annular ridge, the surface of said ridge beingrecessed from the surface of said membrane-depressing face, and saiddisc having at least one gas-escape port connecting said annulardepression to the atmosphere at the rear of said disc; and

b. said supporting structure further comprises a restraining means inthe form of the frustum of a hollow cone, the narrowest portion of thefrustum coacting with said membrane-depressor element in a substantiallyfriction free arrangement to restrict its motion to motion in adirection face.

1. In a pressure sensing head for measuring fluid pressure through amembrane including: a supporting structure containing apressure-development cavity; means for introducing gas at asubstantially fixed rate of flow into said cavity; means for measuringthe gas pressure in said cavity; and a membranedepressor elementsupported by said supporting structure, said membrane-depressor elementhaving a membrane-depressing face with a flow throttling opening in itssurface and a passage connecting said flow throttling opening with saidcavity; the improvement comprising a variable volume feedback chamberwithin said supporting structure formed by said membrane-depressorelement and said pressure-development cavity, said membrane-depressorelement being movably supported by said supporting structure so that thevolume of said feedback cavity is varied by the motion of saidmembrane-depressor element substantially perpendicular to the surface ofsaid membrane.
 2. The pressure sensing head of claim 1 wherein saidsupporting structure comprises a projection coacting with the sides ofsaid membrane-depressor element to restrict its motion to motion in adirection perpendicular to the plane of said membrane-depressing face.3. The pressure sensing head of claim 1 wherein said membrane-depressingface is provided with an annular depression concentric with said flowthrottling opening and separated therefrom by an annular ridge, thesurface of said ridge being recessed from the surface of saidmembrane-depressing face, and said membrane-depressor element isprovided with at least one gas escape port connecting said annulardepression to the atmosphere at a point removed from saidmembrane-depressing face.
 4. The pressure sensing head of claim 1wherein: a. said membrane-depressor element further comprises asubstantially cylindrical support tube supporting saidmembrane-depressing face and b. said supporting structure comprises aprojection coacting with the sides of said cylindrical support tube in asubstantially friction free arrangement to restrict its motion to motionin a direction perpendicular to the plane of said membrane-depressingface.
 5. The pressure sensing head of claim 4 wherein: a. saidmembrane-depressor element further comprises a disc supported by saidsupport tube and containing said membrane-depressing face, saidmembrane-depressing face being provided with an annular depressionconcentric with said flow throttling opening and separated therefrom byan annular ridge, the surface of said annular ridge being recessed fromthe surface of said membrane-depressing face, said disc being furtherprovided with at least one gas-escape port connecting said annulardepression to the atmosphere at the rear of said disc; and b. saidsupporting structure comprises a first body part containing said cavityand a second body part, in the form of the frustum of a hollow cone,said second body part being supported by said first body part and thenarrowest portion of said second body part coacting with said supporttube in a substantially friction free arrangement to restrict its Motionto motion in a direction perpendicular to the plane of saidmembrane-depressing face.
 6. The pressure sensing head of claim 5wherein said support tube comprises a rigid part and a flexible partconnecting said disc to said rigid part, and wherein the second bodypart of said supporting structure comprises a projection coacting withthe rigid part of said support tube to substantially restrict motion ofsaid membrane-depressor element to motion in a direction perpendicularto the plane of said membrane-depressing face.
 7. The pressure sensinghead of claim 1 wherein said supporting structure comprises an annularflange extending from said supporting structure, and a flexiblediaphragm supported in a substantially leaktight arrangement at itsperiphery by said annular flange to form said feedback chamber betweensaid diaphragm and said supporting structure, said membrane-depressorelement being supported by said diaphragm in a substantially leaktightarrangement, and said diaphragm being provided with an openingconnecting said cavity to the passage in said membrane-depressorelement, whereby movement of said membrane-depressor element varies thevolume of said feedback chamber by deformation of said diaphragm.
 8. Thepressure sensing head of claim 7 wherein said membrane-depressor elementcomprises a flexible, substantially cylindrical support tube supportingsaid membrane-depressing face.
 9. The pressure sensing head of claim 8wherein said membrane-depressor element further comprises a rigid sleevesurrounding said flexible support tube over a portion of its length andwherein said supporting structure comprises a projection coacting withsaid rigid sleeve to substantially restrict the motion of saidmembrane-depressor element to motion in a direction perpendicular to theplane of said membrane-depressing face.
 10. The pressure sensing head ofclaim 7 wherein said supporting structure comprises a projectioncoacting with the sides of said membrane-depressor element to restrictits motion to motion in a direction substantially perpendicular to theplane of said membrane-depressing face.
 11. The pressure sensing head ofclaim 7 wherein said diaphragm has an annular deformation circumscribingthe opening in its center, extending away from said feedback chamber,said deformation having an external diameter less than the diameter ofsaid feedback chamber and an internal diameter greater than the externaldiameter of said membrane-depressor element.
 12. The pressure sensinghead of claim 7 wherein: a. said membrane-depressor element comprises adisc containing said membrane-depressing face and a substantiallycylindrical support tube supporting said disc, said membrane-depressingface being provided with an annular depression concentric with said flowthrottling opening and separated therefrom by an annular ridge, thesurface of said ridge being recessed from the surface of saidmembrane-depressing face, and said disc having at least one gas-escapeport connecting said annular depression to the atmosphere at the rear ofsaid disc; and b. said supporting structure further comprises arestraining means in the form of the frustum of a hollow cone, thenarrowest portion of the frustum coacting with said membrane-depressorelement in a substantially friction free arrangement to restrict itsmotion to motion in a direction perpendicular to the plane of saidmembrane-depressing face.